developed showing the ratio between air dry unit weight and fresh concrete
unit weight for each mix.
1.9.4
Average Compressive Strength Required for Mixtures
The mixture proportions selected during mixture design studies shall
produce a required average compressive strength (f'cr) exceeding the
specified compressive strength (f'c) by the amount indicated below. This
required average compressive strength, f'cr, will not be a required
acceptance criteria during concrete production. However, whenever the
daily average compressive strength at 28 days drops below f'cr during
concrete production, or daily average 7-day strength drops below a strength
correlated with the 28-day f'cr, the mixture shall be adjusted, as
approved, to bring the daily average back up to f'cr. During production,
the required f'cr shall be adjusted, as appropriate, based on the standard
deviation being attained on the job.
1.9.4.1
Computations from Test Records
Where a concrete production facility has test records, a standard deviation
shall be established in accordance with the applicable provisions of ACI
214.3R. Test records from which a standard deviation is calculated shall
represent materials, quality control procedures, and conditions similar to
those expected; shall represent concrete produced to meet a specified
strength or strengths (f'c) within 7 MPa 1,000 psi of that specified for
proposed work; and shall consist of at least 30 consecutive tests. A
strength test shall be the average of the strengths of two cylinders made
from the same sample of concrete and tested at 28 days. Required average
compressive strength f'cr used as the basis for selection of concrete
proportions shall be the larger of the equations that follow using the
standard deviation as determined above:
f'cr = f'c + 1.34S where units are in MPaf'cr = f'c + 1.34S where units are
in psi
f'cr = f'c + 2.33S - 3.45 where units are in MPaf'cr = f'c + 2.33S - 500
where units are in psi
Where S = standard deviation
Where a concrete production facility does not have test records meeting the
requirements above but does have a record based on 15 to 29 consecutive
tests, a standard deviation shall be established as the product of the
calculated standard deviation and a modification factor from the following
table:
MODIFICATION FACTOR
NUMBER OF TESTS
FOR STANDARD DEVIATION
15
1.16
20
1.08
25
1.03
30 or more
1.00
1.9.4.2
Computations without Previous Test Records
When a concrete production facility does not have sufficient field strength
test records for calculation of the standard deviation, the required
average strength f'cr shall be determined as follows:
SECTION 03300A
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